Pin pysnmp version to 4.4.12

[davidvng]

Description

Pin pysnmp==4.4.12

Motivation and Context

PyPi of pysnmp recently became the same as pysnmp-lextudio which removed twisted.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.